The presence of sulphur in steel has a detrimental effect on the surface quality and degree of cracking, and consequently on the degree of scarfing and grinding required and the yield achieved in the finishing process. Particularly in the fields of brittle failure, welding and fabrication, steel requirements are becoming increasingly stringent, and sulphur can play an important and sometimes dominant role through its effect on steel properties.
Thus, the ever increasing emphasis on steel quality combined with the widening use of automatic production methods compel steelmakers to reduce sulphur levels to the absolute minimum that is economically practical.
With the exception of the double slag electric arc process, ultra low sulphur steels below 0.01% cannot be produced by conventional steelmaking processes unless special attention is given to ensure that the charge materials are low in sulphur. Difficulties in achieving sulphur specifications due to the variable quality of these charge materials (and particularly steel scrap) often result in extra furnace time with the subsequent increase in steelmaking costs and reduction in productivity. The increasing commercial demand for very low sulphur steels has resulted in the adoption of means for the external desulphurization of molten iron, i.e. by means outside the blast furnace.
Several methods are already used for effecting external desulphurization which include stirring a desulphurizing agent such as calcium carbide into the metal, plunging desulphurizing agents, e.g. magnesium impregnated into coke under the surface of the molten metal and injecting particulate desulphurizing agents, e.g. lime, calcium carbide, or calcium cyanamide into the metal.
All the methods mentioned above involve the desulphurization of molten ferrous metal by treating the impure metal, in a suitable vessel, with an agent having known desulphurization properties. The vessel in question may be, for example, a transfer ladle or holding ladle, a mixer vessel in which molten iron from the blast furnace is stored before conversion into steel, or a torpedo ladle.